While Android's market share rose from 46.9 percent in November, it is iOS that finished the year on a strong note. The platform topped 29.6 percent through December, up nearly a full percentage point from 28.8 percent in November.

The growth spurt came courtesy of two and a half months of sales of the popular iPhone 4S on AT&T (NYSE:T), Verizon Wireless and Sprint (NYSE:S).

In third place, Research In Motion's (NASDAQ:RIMM) BlackBerry platform continued its slide, ending 2011 with 16 percent share, down from nearly 20 percent in August. The company is betting on its BlackBerry 10 operating system to help sell more phones.

Microsoft (NASDAQ:MSFT) Windows Mobile/Phone market share also continued to slide. Windows Mobile/Phone capped 2011 with 4.7 percent market share in the U.S., down from 5.2 percent in November and 5.7 percent share in August 2011. However, the company's Windows Phone 7 appears to be trending up on new Nokia (NYSE:NOK) Lumia phones. It is widely expected Windows Phone will become the No. 3 smartphone platform behind Android and iOS in the next couple years.

Overall, smartphone penetration continued its ascendance through the end of 2011. comScore counted about 98 million smartphone users in the U.S. through December, accounting for 40 percent of all U.S. mobile users, and up from 91.4 percent in November.